Ultra-strong interaction of metamaterial plasmons with photons in a terahertz photonic crystal cavity

腔(量子)电动力学研究共振腔中物质与光子的强相互作用。长期以来，人们一直在深入研究物理学中的基本量子性质，如薛定谔猫态和光子纠缠，并探索量子信息处理的方法。最初，强耦合只被研究与自然原子，但最近几年，已在各种材料系统中实验实现，利用半导体量子阱和量子点的带间和亚带间跃迁，磁性材料的自旋共振，以及玻色子激发，如二维电子气中的回旋跃迁和聚合物中的分子振动跃迁。我们最近在一维光子晶体腔中展示了一类人造材料亚表面与太赫兹光子的强光-物质耦合。我们使用了等离子体金属结构，这种结构在文献中得到了广泛的研究，尤其是因为它们在实现化学和生物传感器、光学滤光器和调制器方面的潜力。利用它们巨大的偶极矩，变表面的单胞与腔内的光子有效地相互作用，形成等离子体-光子极化子，上下极化子分支发生巨大的拉比分裂。尽管这是一个经典的电动力学系统，但事实证明，腔量子电动力学中使用的术语是有用和适用的。基于我们的发现，我们打算进行两条线的研究。一个是致力于更好地理解和加强这种形式的光-物质相互作用。我们想要回答关于巴比奈互补亚曲面与光子之间出乎意料的强耦合的公开问题，然后研究超材料中通过单元间相互作用与腔光子的耦合。为了实现更强的耦合--达到超强耦合区域的高度--我们接着专注于太赫兹法布里-珀罗腔的发展，它比我们目前所研究的光子晶体腔具有更小的基模体积。随着模式体积的减小，我们的目标是展示比目前所获得的更强的极化子分支的拉比分裂。在第二条工作线上，我们开发了主动切换互动强度的能力。文献已经展示了通过外部控制参数来改变亚表面的光学性质的方法。在腔内的强相互作用下，这应该会工作得更好。我们将专注于从裂环谐振器构建的准表面，并采用概念来改变它们的性质，通过施加到单位电池的偏置电压，或通过吸收照射到它们上面的激光辐射。通过这项研究，我们打算为未来的应用准备交换平台。

Cavity (quantum) electrodynamics investigates the strong interaction of matter with photons in a resonant cavity. It has long been studied intensively to investigate fundamental quantum properties in physics such as Schrödinger's cat states and entanglement of photons, and to explore approaches for quantum information processing. Initially, strong coupling was only investigated with natural atoms, while in recent years, it has been realized experimentally in a variety of material systems, employing interband and inter-subband transitions in semiconductor quantum wells and quantum dots, spin resonances in magnetic materials, and bosonic excitations such as cyclotron transitions in 2D electron gases and molecular vibrational transitions in polymers. We have recently demonstrated strong light-matter coupling of metasurfaces, a class of artificial materials, with terahertz photons in a one-dimensional photonic crystal cavity. We have employed plasmonic metallic structures which have been studied extensively in the literature, not least because of their potential for the realization of chemical and biological sensors, optical filters and modulators. With their huge dipole moments, the unit cells of the metasurfaces interact efficiently with the photons in the cavity, forming plasmon-photon polaritons with an enormous Rabi splitting of the upper and lower polariton branches. In spite of this being a classical electrodynamic system, it has turned out that the terminology employed in cavity quantum electrodynamics is useful and applicable.Based on our findings, we intend to pursue two lines of research. One is devoted to a better understanding and an enhancement of this form of light-matter interaction. We want to answer open questions regarding the unexpectedly strong coupling of Babinet-complementary metasurfaces with the photons, and then study the coupling of ‘dark atoms’ with the cavity photons via inter-unit-cell interaction in the metamaterial. In order to achieve even stronger coupling – reaching high into the ultrastrong-coupling regime – we then focus on the development of terahertz Fabry-Perot cavities which exhibit a smaller fundamental-mode volume than the photonic crystal cavities studied by us so far. With this reduction of the mode volume, we aim to demonstrate an even stronger Rabi splitting of the polariton branches than obtained until now. In the second line of work, we develop capabilities to actively switch the interaction strength. The literature has shown ways to modify the optical properties of metasurfaces by external control parameters. This should work even better with the strong interaction in a cavity. We will focus on metasurfaces built from split-ring-resonators, and adopt concepts to change their properties by a bias voltage applied to the unit cells, or by the absorption of laser radiation impinging on them. With this research, we intend to prepare switching platforms for future use in applications.